San Francisco is hoping to better prepare for the next drought. Though the city’s government agencies were good at cutting water use during the recent dry years, easily meeting a self-imposed goal of reducing consumption 10 percent between 2014 and 2017 and often conserving more, Mayor Mark Farrell wants to pick it up a notch. Farrell is asking the Board of Supervisors to approve an ordinance that would require the five city departments that use the most water to develop plans for trimming water use 20 percent.